Acquiring samples representative of downhole fluids is an important aspect of determining the economic value of an hydrocarbon formation. However where a volatile gas is dissolved in the fluid, the sample taken downhole may under-represent the proportion of the volatile gas within the fluid due to its reaction with the material from which sampling apparatus is made. This leads to an underestimate of the proportion of volatile gas.
There is a particular problem where hydrogen sulphide (H.sub.2 S) is dissolved in the fluid. H.sub.2 S is highly corrosive and toxic and any underestimate of the proportion of this gas within the fluid can affect the economics of well production, as pipework will need to be replaced sooner than expected. Underestimate of the presence of corrosive gases such as H.sub.2 S is having an effect on the economies of well production. In the last decade, it here has been observed that the proportion of H.sub.2 S within hydrocarbon formations is increasing, partly as a result of accessing deeper formations.
Attempts to address this problem have included coating metal sampling apparatus with inert layers or the use of glass bottles to collect the samples of fluid. However these are relatively expensive and prevent the use of existing hardware. It is an objection of the present invention to provide apparatus and method for sampling downhole fluid which reduces the underestimation of volatile gases in hydrocarbons.